The aforedescribed copending application deals with large-diameter hollow bodies, especially waste and sewer pipe, for subterranean use whereby deflection or bending of the pipe is possible within the elastic limit of the synthetic-resin material so that, with time, the deformation stresses are eliminated by a relaxation process. The instant application deals with an apparatus for the making of such pipe and it is to be understood that the product of the present method and apparatus is any one of the synthetic-resin ducts or tubes described in the aforementioned copending application.
In the manufacture of pipe, helical-seam techniques have gained increasing interest. For example, steel or other metal strip may be fed tangentially to a cylindrical mandrel at an angle to the axis thereof so that the strip is coiled in a multiplicity of successive turns on the mandrel with confronting edges of the successive turns being welded together by deposit-welding, i.e. the formation of a weld bead or weldment along the helical seam between the turns.
Such a system permits large-diameter ducts to be made with substantially unlimited length, using a strip of narrow width and unlimited length.
With the advent of synthetic resins, considerable effort was invested in developing an analogous system for the production of thermoplastic synthetic-resin pipes and there have been described in the literature, systems in which strips of synthetic resin were coiled so that overlapping flanges were formed and heat-sealed or otherwise bonded together. The fusion pressure is thus applied in the radial direction.
Pipe of this type has insufficient flexibility for subterranean disposition as a waste pipe. The radial pressure has effectively precluded the use of hollow strips for this purpose, at least where an internal passage is to be maintained in the strip.